This invention relates generally to methods and apparatus for eroding or ablating surfaces by lasers. In particular, this invention relates to methods and apparatus for eroding surfaces astigmatically and for the correction of myopic astigmatism through laser keratoplasty or keratomileusis.
Lasers can etch surfaces in a controlled manner. These techniques are known and depend upon the interaction between the surface structure and the laser radiation which has a known wavelength and energy density. In addition, the ablation of a surface by laser radiation is typically time-rate dependent, although this time dependency is usually non-linear with ablative depth because of heat build-up and other artifacts. Therefore, ablative precision is often obtained through the use of pulsed laser radiation. Short pulses provide controlled depth etching in the application area. This control is especially important in keratoplasty or in keratomileusis procedures where the cornea of the eye is ablated to correct certain visual deficiencies, such as near-sightedness. By careful application of laser radiation to the cornea, physicians are able to surgically enhance a patient's vision with precision and without tissue damage by overexposure.
By successive application of laser pulses to a surface and by altering the size of the area under irradiation, curvatures can be created or altered on a surface. For example, when the ablative technique begins by irradiating a large, circular area and progressively decreases the radius of the exposed area, the central region has the greatest ablative depth because it is irradiated the longest. Conversely, the outer-most regions have the least ablative depth. If the surface is initially flat, the resulting profile would be concave. If the surface is initially convex, the surface can be made flat, or the curvature can be reduced depending upon the degree of ablation. In keratoplasty or in keratomileusis, the curvature on the cornea, i.e., the difference between the cornea's ideal curvature and the patient's actual corneal shape, is modified.
Surface erosion techniques to create or remove astigmatic shapes have been less successful. An astigmatic surface is by definition bi-powered: there are two natural and orthogonal curvatures creating the surface. A varying, circular irradiation pattern can only create or remove a single power on a surface. Existing methods to erode astigmatic shapes have been complex and difficult to implement.
For example, U.S. Pat. No. 4,665,913 entitled "Method For Ophthalmological Surgery" discloses methods which provide for different astigmatic shapes through the use of laser scanning. However, this approach is especially difficult to control. The precise control of a reprofiling operation on a surface, e.g., a cornea, using a scanning laser, requires that the laser spot maintain nearly constant spatial intensity and further that the complex etching pattern is carefully followed. However, laser spot intensities are mostly Gaussian and inherently non-uniform. This non-uniformity and the detailed etching pattern leave much room for error, and, thus, require expensive safeguards for surgical applications. Additionally, these methods are naturally time-consuming, since only a small portion of the surface is ablated at a given time.
Another method for eroding an astigmatic surface is provided in U.S. Pat. No. 4,941,093 entitled "Surface Erosion Using Lasers." According to its teachings, specifically configured optical elements or slits are used to provide ablation in one axis, i.e., such that the erosion proceeds selectively relative to a line rather than around a point. Typically, this approach requires a second step to provide spherical correction so that the proper overall curvature is achieved.
Yet another approach involves the use of a graded intensity or photodecomposable mask which varies the laser transmission to the target surface, thereby inducing variable ablative depths on the surface. For example, U.S. Pat. No. 4,856,513 entitled "Laser Reprofiling Systems And Methods" which describes methodology for selectively eroding the cornea through the use of an erodable mask. The mask absorbs the surface laser radiation in varying amounts across the corneal surface to provide the desired surface profiles. This technique though requires the manufacture of a complementary object, i.e., the erodable mask, and also requires precise correlative positioning over the target surface.
It is, accordingly, an object of this invention to provide a simpler method and apparatus for astigmatically reprofiling a surface with an initial, bi-powered, astigmatic shape in order to achieve a new, preferably spherical, shape.
It is another object of this invention to provide a method and apparatus for orienting and adjusting the astigmatic ratio applied to a surface shape with increased control.
It is further an object of this invention to provide a method and apparatus for correcting myopic astigmatism through corneal ablation in laser keratomileusis procedures.
These and other objects of the invention are evident in the description that follows.